During November 2008 the eminent neurologist Dr Oliver Sacks of Columbia University and Michael Boyd of the Royal Shakespeare Company took part in a discussion looking at the phenomenal memory of actors.

Boyd began by describing how, over a three year period, he had worked with a group of 30 actors performing the series of Shakespeare's plays known as The History Plays. To complete this feat, each actor needed to perform in at least seven plays, remembering not only thousands of lines, but also their location on the stage, the cues of other performers and the required emotional state.

The process began with each actor learning their lines in isolation. They then progressed to sitting together reading the plays out loud. However, they found that the actors were struggling to learn their lines with these two processes so they decided to act out the parts on an empty stage - without costumes or props. What they found was that the actors were almost instantly word perfect. As Boyd put it; "It was clear that what they were trying to retrieve was no more than a broken bit of memory, only complete when the actions of their bodies and the emotions were combined together with the recall of the line." "I think that says something about where we keep our memory. Maybe our memory is in our body as well as in our cranium."

Michael Boyd's observation is similar to the findings of a research team who studied the brains of London taxi drivers in 2000 and found that they could not remember where a street was unless they "physicalised" the journey to that street mentally through the physical sense of turning left and turning right.

Oliver Sacks responded by pointing out that memory and performing are two different things. He cited as an example the case of a musician who had his hippocampus systems wiped out by an encephalitis 20 years ago. As a result, he cannot remember things for more than a few seconds yet he is still able to conduct a choir, conduct an orchestra, play the piano or sing long, complex pieces of music.

The point is that what we simplistically refer to as "memory" is in actual fact a complex set of mental processes that link with our senses, our values and our historic experiences. That is why it is easier to learn something that resonates with us for other reasons than it is to learn some anodyne fact that has no meaning, significance or importance to us - it's as though those memories, feelings and emotions "light up" in sympathy with the new knowledge.

When an actor learns their lines they are not simply learning a sequence of words, they are becoming the person they are playing and sensing their being and emotional state.

Oliver Sacks described how when they were working on the film Awakening (in which Robin Williams played Sacks), Williams would spend all his time with Sacks observing him. Sacks then noticed that over time Williams was adopting the same posture and poses, that he was mirroring his gestures, style of speech and other idiosyncrasies. Sacks described it as being like having a younger twin.

Ironically therefore, in having to learn lines, cues, emotions, expressions and stage positioning makes learning easier than if all you had to remember were your lines. As Boyd put it; "You are not trying to recall what you are doing, you are simply in the act of doing it."

A recording of the discussion can be viewed by clicking here (requires QuickTime) or an edited transcript of the discussion can be obtained here.

Michael Boyd is artistic director of the Royal Shakespeare Company; Oliver Sacks MD is professor of neurology and psychiatry, Columbia University Medical Center, NYC. The discussion was chaired by Lee C Bollinger, president of Columbia University.